DE3852532T2 - Liquid crystal display device. - Google Patents

Description

The invention relates to a multilayer liquid crystal display device using a twisted nematic display process, by which a black/white display image and a color display image with high contrast can be produced.

Liquid crystal display devices are now used in watches and electronic computing devices, computer terminals, word processing displays, televisions and a variety of other uses in many fields. Recently, extremely large demand has arisen for liquid crystal display devices due to changes to multi-color and full-color displays, which are already used in the fields of graphic display and image display. Color displays, which have been put into widespread practical use, are achieved by liquid crystal cells having color filter layers. A liquid crystal cell works as a light switch and produces various colors. The main type of display mode is the twisted nematic (TN) display mode, which is achieved by a liquid crystal cell in which the liquid crystal molecules are twisted by 90º, whereby high contrast, etc., can be achieved. However, in this TN display mode, the dependence of the display characteristics on the wavelength of light is large, and it is not possible to achieve uniform switching of light over the entire spectrum of visible light. In particular, in a display process with blocking in the normal state in which the absorption axes of the two polarizers are parallel, there is a difficulty that light leakage leads to color when the voltage is zero.

In a color display device that performs light switching using a TN display having this type of color filter layer, there are two main types of driving methods. One of these involves an active matrix driving method that uses a liquid crystal cell having picture elements provided with nonlinear devices such as diodes or switching elements such as thin film transistors. The other method is a multiplex driving method in which the liquid crystals of a liquid crystal cell are sequentially driven without switching elements. In the latter method, the slope near the threshold of the optical properties of the liquid crystal is important; this is a difficulty in TN displays as currently used. In order to improve the optical characteristics so that steepness can be achieved near the threshold, a super-twisted birefringence (SBE) process has been proposed, which results from liquid crystal molecules twisted at angles of approximately 180º - 270º. In the SBE process, the curve rises steeply near the threshold, and even if the duty ratio increases, it is possible to achieve a high contrast ratio. However, since birefringence effects of liquid crystals are used, the dependence of the display characteristics on wavelength is theoretically larger than that of a TN display, so it is very difficult to achieve adaptation for use in a full-color display.

It is an object of the invention to provide a liquid crystal display device which overcomes the above-discussed and other disadvantages and deficiencies in the prior art.

From the earlier UK application GB-A-2 092 769 a positive type liquid crystal display device is known, comprising: a liquid crystal cell having a first and a second cell layer containing liquid crystal molecules with respective twisted nematic alignments, the liquid crystal molecules of the first cell layer having a twist angle opposite to the twist angle of the liquid crystal molecules in the second cell layer, respective alignment layers being associated with the first and second cell layers; and voltage applying means operatively connected to the first cell layer for applying a voltage for changing the alignment of the liquid crystal molecules in the first cell layer, the twist alignment of the liquid crystal molecules in the second cell layer being unaffected by the voltage applying means.

From EP-A-0 131 216 a single-layer twisted nematic liquid crystal display device with twist angles in the range 180º - 360º is also known, in which the polarization state of light entering a cell layer is determined by adjusting the polarization axis of a polarizing plate in order to thereby achieve a significant change in the optical properties.

According to the invention there is provided a positive type liquid crystal display device comprising:

- a multilayer liquid crystal cell comprising a first and a second cell layer, the cell layers containing liquid crystal molecules with twisted nematic alignment;

- wherein the first cell layer comprises a pair of transparent substrates facing each other, a first pair of alignment films formed on the inner sides of the transparent substrates, wherein the liquid crystal molecules of the first cell layer are arranged with twisted nematic alignment between the first pair of alignment films, and includes a voltage applying device capable of changing the direction of the long molecular axes of each of the liquid crystal molecules of the first cell layer;

- wherein the second cell layer includes a pair of transparent substrates facing each other and a second pair of alignment films formed on the inner sides of the latter transparent substrates, the liquid crystal molecules of the second cell layer having twisted nematic alignment being present between the second pair of alignment films; and

- wherein the twist direction of the liquid crystal molecules in the first cell layer is reverse to the twist direction of the liquid crystal molecules in the second cell layer;

- wherein the twist angle of the liquid crystal molecules in the first cell layer and the twist angle of the liquid crystal molecules in the second cell layer are in the range of 180º to 360º and

- wherein the angle formed between the alignment direction along the surface of the alignment film of the longer axis of the liquid crystal molecules closest to the surface of this alignment film of the first cell layer closest to the second cell layer and the alignment direction along the surface of the alignment film of the longer axis of the liquid crystal molecules closest to the surface of this alignment film of the second cell layer closest to the first cell layer is between 70º and 80º or between 110º and 150º.

In a preferred embodiment, the angle between the alignment directions is between 120º and 150º.

In a preferred embodiment, the twist angles of the liquid crystal molecules in the first and second cell layers are almost equal to each other and the product Δn₁ d₁ of the birefringence Δn₁ and the thickness d₁ of the liquid crystal layer in the first cell layer and the product Δn₂ d₂ of the birefringence Δn₁ and the thickness d₂ of the liquid crystal layer in the second cell layer are represented by the following inequality:

In a preferred embodiment, the relationship between the pitch p of the twist of the liquid crystal molecules in the first cell layer and the thickness d of the liquid crystal layer in the first cell layer is as follows:

where Θ is the twist angle of the liquid crystal molecules.

In a preferred embodiment, a color filter layer is disposed in at least one of the first and second cell layers.

In a preferred embodiment, the first cell layer contains pixels, with an active device arranged at each pixel.

Thus, the invention described here makes it possible to provide: (1) a positive type liquid crystal display device which can produce a black/white display image and a color display image with excellent color reproduction and high contrast; and (2) a positive type liquid crystal display device that achieves full-color display or multi-color display.

The invention is described below by way of example only with reference to the accompanying drawings in which:

Fig. 1 is a cross-sectional view showing the basic cell structure of a multilayer liquid crystal display device according to the invention;

Fig. 2a and 2b are diagrams showing the twisting of liquid crystal molecules to the right and left, respectively;

Fig. 3 is a characteristic curve showing the relationship between the alignment angle of the liquid crystal molecules in the first cell layer near the second cell layer to the alignment of the liquid crystal molecules in the second cell layer near the first cell layer and the contrast ratio;

Fig. 4 is a characteristic curve showing the relationship between the twist angle of liquid crystal molecules and the contrast ratio in display images;

Fig. 5 is a characteristic curve showing the relationship between the value Δn₁ d₁/Δn₂ d₂ and the contrast ratio ;

Fig. 6a are characteristic curves showing the relationship between the applied voltage and the light transmittance for the display device according to the invention shown in Fig. 1 ;

Fig. 6b Characteristic curves showing the relationship between the applied voltage and the light transmittance for a single-layer TN LCD;

Fig. 7a are characteristic curves showing the relationship between the applied voltage and the light transmittance for other multilayer display devices according to the invention; and

Fig. 7b are characteristic curves showing the relationship between the applied voltage and the light transmittance for another single-layer TN-LCD.

The invention provides a liquid crystal display device whose basic double-layer cell structure consists of a first cell layer C1 and a second cell layer C2 as shown in Fig. 1, which contain liquid crystal molecules having twisted nematic alignment. The first cell layer C1 comprises transparent substrates 1 made of glass, acrylic resin or the like, transparent conductive films 2 made of ITO, NESA film or the like provided on the substrates 1, and alignment films 3 made of an inorganic film made of SiO2, SiO or the like or an organic film made of polyimide, polyvinyl alcohol, nylon, acrylic resins or the like for aligning the liquid crystal molecules, which are provided on the substrates 1 and the transparent conductive films 2. A polarizer 5a is arranged on the back of the substrate 1 of the second cell layer C2, and a detector 5b is arranged on the back of the substrate 1 of the first cell layer C1. Both ends of each cell layer are sealed by sealing substances 6. A liquid crystal layer 4 is arranged in each of the cell layers C1 and C2. The second cell layer C2 is substantially the same as the cell layer C1 except that it does not have the transparent conductive films 2.

The direction in which the liquid crystal molecules of the liquid crystal layer 4 in one cell layer are helically twisted is opposite to the twisting direction of the liquid crystal molecules of the liquid crystal layer 4 in the other cell layer. The twisting directions of the liquid crystal molecules are as shown in Figs. 2a and 2b, wherein Fig. 2a shows the twisting of the liquid crystal molecules to the right with respect to the direction in which the light from a light source is incident on the cell, and Fig. 2b shows the twisting of the liquid crystal molecules to the left with respect to the direction of light incidence. When an optically active substance is added to a nematic liquid crystal, the liquid crystal molecules form a twisted structure. In order to cause the liquid crystal molecules to undergo a twist to the right, the one represented by the following chemical structure is used as the optically active substance:

To make the liquid crystal molecules twist to the left, cholesteryl nonanoate (Merck), S-811 (Merck), etc. are used as optically active substances.

Voltage is applied to the liquid crystal layer 4 of the first cell layer C₁ through the transparent conductive films 2, whereby the alignment of the liquid crystal molecules of this first cell layer C₁ can be changed. Since no transparent conductive films 2 are disposed on the liquid crystal layer 4 of the second cell layer C₂, no change in the orientation of the liquid crystal molecules of the second cell layer C₂ occurs. The second cell layer C₂ serves as a compensator which makes the display pattern of the first cell layer C₁ clear.

Fig. 3 shows the relationship between the angles α of the alignment of the liquid crystal molecules in the first cell layer C₁ closest to the second cell layer C₂ and the alignment of the liquid crystal molecules of the second cell layer C₂ closest to the first cell layer C₁ and the contrast ratio, where the twist angles Θ₁ and Θ₂ of the liquid crystal molecules in the liquid crystal layers in the first and second cell layers C₁ and C₂ are set as follows: Θ₁ = Θ₂ = 270°, and the values Δn₁ d₁ and Δn₂ d₂ (Δn₁ and Δn₂ correspond to the birefringence of the liquid crystals in the first and second cell layers, respectively, and d₁ and d₂ are the thicknesses of the liquid crystal layers in the first and second cell layers, respectively) of the liquid crystal layers in the first and second cell layers are set as follows: Δn₁ d₁ = Δn₂ d₂ = 0.7. It can be seen from Fig. 3 that the value of α should be set to 70° to 150° in order to obtain a contrast ratio that is 1/2 times or more of the maximum contrast ratio.

When the value of α is in the range of 70º to 80º or 100º to 150º, due to the birefringence of the liquid crystal in the second cell layer, the first cell layer achieves high light transmittance when the voltage is zero and it achieves low light transmittance when the voltage is applied; that is, the first cell layer gives a positive display. When the value of α is in the range of 80º to 100º, the first cell layer gives the opposite effect, i.e. a negative indication.

Since it is desired that reflection type liquid crystal display devices give a positive display, the value of α should be set to 70º - 80º or 100º - 150º. In view of the manufacturing ease, it is preferably set to 120º - 150º.

The liquid crystal layers are designed in such a way that the following relationship can be fulfilled, since maximum contrast ratio is present:

When the display contrast and visibility are taken into account, as shown in Fig. 4, the twist angle of the liquid crystal molecules is set in the range of about 180° to about 360° based on the relationship between the twist angle and the contrast ratio. When the twist angle of the liquid crystal molecules exceeds 360°, a domain occurs in which the alignment of liquid crystals is disordered upon application of voltage, resulting in light scattering, which easily leads to a reduction in contrast.

Fig. 5 shows the relationship between the value of (Δn1 d1)/(Δn2 d2) and the contrast ratio, where α = 110° and Θ1 = Θ2 = 270°, indicating that the value of (Δn1 d1)/(Δn2 d2) should be set to be larger than 0.7 and smaller than 1.4 (0.7 < (Δn1 d1)/(Δn2 d2) < 1.4) in order to achieve a contrast ratio that is 1/2 times or more of the maximum contrast ratio. The absolute value of each of Δn₁, d1, Δn₂, and d₂ is preferably set to be in the range of 0.3 to 3.0.

When the liquid crystal device is driven by the multiplex driving method by a voltage applying means in order to obtain a sharp threshold characteristic for contrast, the specific twist pitch p of the liquid crystal molecules becomes very important. The ratio of the thickness d of the liquid crystal layer to the twist pitch p of the liquid crystal molecules, d/p, is preferably set using experimental data as follows:

where Θ is the twist angle of the liquid crystal molecules. This requirement applies when the pretilt angle of the liquid crystals is approximately 10º or less.

example 1

Fig. 1 shows a double-layered cell structure of a liquid crystal display device (i.e., a double-layered TN-LCD) according to the present invention, in which the transparent substrate 1 is made of glass. A transparent conductive film 2 having a thickness of about 1500 Å is deposited on each of the glass substrates 1 of the first cell layer C1 alone by vapor deposition of ITO and patterned by an etching technique. On the glass substrate 1 and the transparent conductive films 2, liquid crystal molecule alignment films 3 made of polyimide having a thickness of about 1000 Å are formed by a spin coating method, the surface of which is treated by rubbing with a cloth, causing the liquid crystal molecules to be in twisted nematic alignment. The end portions of the cell layers are sealed by a sealing substance 6.

The value of α is set to 135°. As the liquid crystal substance, nematic liquid crystal ZLI-3281 (Merck) is used. 0.94 wt% of CB15 is added to the liquid crystal layer 4 of the first cell layer C₁, and 1.1 wt% of cholesteryl nonanoate is added to the liquid crystal layer 4 of the second cell layer C₂. The twist angle of the liquid crystal molecules in the first cell layer C₁ is opposite to that of the liquid crystal molecules in the second cell layer C₂, where θ₁ = θ₂ = 270°. The thickness of the liquid crystal layer in both the first and second cell layers C₁ and C₂ is (i.e., the thickness of each of the cell layers C1 and C2) is 5 µm, i.e., d1 = d2 = 5 µm. P is approximately 8 µm. The pretilt angle of the liquid crystal molecules on the substrate 1 is approximately 8º. The polarizer 5a and the detector 5b, which consist of an iodine system polarizing plate, are arranged at a mutual angle of approximately 45º. Figures 6a and 6b show the dependence of light transmittance on applied voltage for a double-layer TN-LCD according to the invention and for a standard single-layer TN-LCD as a reference, respectively, with the wavelengths used for red, green and blue colors being 610 nm, 550 nm and 450 nm, respectively, and it can be seen that when a voltage is applied close to the threshold value, the transmittance for the wavelength for each color in the double-layer cell is lower than for the wavelength for each color in the single-layer cell. This means that the double-layer cell can achieve higher contrast. Furthermore, the wavelength dependence of the application voltage/transmittance characteristic for the double-layer cell is much smaller than the wavelength dependence of the application voltage/transmittance characteristic for the single-layer cell and accordingly the double-layer cell can produce a clear black/white positive-type display image.

Example 2

This example provides another liquid crystal display device according to the present invention, which has the same structure as that of Example 1, except that 0.75 wt% of CB15 is added to the liquid crystal layer 4 of the first cell layer C1 and 0.84 wt% of cholesteryl nonanoate is added to the liquid crystal layer 4 of the second cell layer C2, and furthermore, the twist angles Θ1 and Θ2 of the liquid crystal molecules in the first and second cell layers C1 and C2 are each set to 240° (i.e., Θ1 = Θ2 = 240°). Fig. 7a shows the dependence of light transmittance on applied voltage for the double-layer TN-LCD of the present invention, and Fig. 7b shows the same characteristic as just mentioned for a standard single-layer TN-LCD for reference, where the wavelengths λ studied here are 610 nm for red, 550 nm for green, and 450 nm for blue. Figs. 7a and 7b show that the transmittance characteristics at the wavelengths for red, green, and blue for the double-layer cell are more uniform than those for these wavelengths for the single-layer cell, so that a clear black-and-white positive-type display image can be obtained. In addition, this double-layer cell has a sharp threshold characteristic so that the cell can produce a high-contrast display image.

Example 3

Inside the liquid crystal cell layer with voltage applying means in each of the display devices of the above examples, color filter (red, green and blue) layers made of a gelatin film are provided. The display devices with the color filter layers are subjected to subjected to multiplexing, producing a clear, crisp color image. These liquid crystal display devices are suitable for full-color display and multi-color display.

Example 4

Liquid crystal cells according to the invention are used in place of a liquid crystal panel with color filters provided with a TFT (thin film transistor) as an active device, and are examined for color display experiments in active matrix drive. They produce a clear and distinct color image, so that they are useful for full-color display and multi-color display.

It is to be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims (6)

1. A positive type liquid crystal display device comprising: - a multilayer liquid crystal cell comprising a first and a second cell layer (C₁, C₂), wherein the cell layers contain liquid crystal molecules with twisted nematic alignment; - wherein the first cell layer (C₁) includes a pair of transparent substrates (1, 1) facing each other, a first pair of alignment films (3, 3) formed on the inner sides of the transparent substrates (1, 1), the liquid crystal molecules of the first cell layer having twisted nematic alignment are arranged between the first pair of alignment films (3, 3), and a voltage applying device capable of changing the direction of the long molecular axes of each of the liquid crystal molecules of the first cell layer; - wherein the second cell layer (C₂) includes a pair of transparent substrates (1, 1) facing each other and a second pair of alignment films (3, 3) formed on the inner sides of the latter transparent substrates (1, 1), the liquid crystal molecules of the second cell layer having twisted nematic alignment being present between the second pair of alignment films (3, 3); - wherein the twist direction of the liquid crystal molecules in the first cell layer (C₁) is reverse to the twist direction of the liquid crystal molecules in the second cell layer (C₂); characterized in that - both the twist angle of the liquid crystal molecules in the first cell layer (C₁) and the twist angle of the liquid crystal molecules in the second cell layer (C₂) are in the range of 180º to 360º and - the angle between the alignment direction along the surface of the alignment film (3) of the longer axis of the liquid crystal molecules closest to the surface of this alignment film (3) of the first cell layer (C₁) closest to the second cell layer (C₂) and the alignment direction along the surface of the alignment film (3) of the longer axis of the liquid crystal molecules closest to the surface of this alignment film (3) of the second cell layer (C2) closest to the first cell layer (C₁) is between 70º and 80º or between 110º and 150º. 2. A liquid crystal display device according to claim 1, wherein the angle between the alignment directions is between 120° and 150°. 3. A liquid crystal display device according to claim 1, wherein the twist angles of the liquid crystal molecules in the first and second cell layers (C₁, C₂) are almost equal to each other, and the product Δn₁ d₁ of the birefringence Δn₁ and the thickness d₁ of the liquid crystal layer in the first cell layer (C₁) and the product Δn₂ d₂ of the birefringence Δn₁ and the thickness d₂ of the liquid crystal layer in the second cell layer (C₂) are represented by the following inequality: 4. A liquid crystal display device according to claim 1, wherein the relationship between the pitch p of the twist of the liquid crystal molecules in the first cell layer (C₁) and the thickness d of the liquid crystal layer in the first cell layer (C₁) is as follows: where Θ is the twist angle of the liquid crystal molecules. 5. A liquid crystal display device according to any one of the preceding claims 1 to 4, wherein a color filter layer is arranged in at least one layer of the first and second cell layers (C₁, C₂). 6. A liquid crystal display device according to any one of the preceding claims 1 to 5, wherein the first cell layer contains picture elements and wherein an active component is arranged on each picture element.